The 43 Chipotle restaurants in Washington and Oregon that were closed after an outbreak of E. coli are due to reopen. Dozens were made sick by the outbreak that spread across both states. The source of the outbreak still has not been found, though health officials connect the cases to the restaurant.

ABC News reports that 27 people in Washington and 15 in Oregon were infected with E. coli. According to health officials, most individuals that were affected reported that they had eaten at a Chipotle restaurant before they became sick.

After an extensive cleaning and testing process Chipotle replaced some of its ingredients and announced this week that the closed locations would reopen. Washington health officials are still performing tests, but do not feel that an exact source of the outbreak will be discovered.

In a statement the Washington Department of Health said that this can happen when “the contaminated food source was consumed before the food samples were collected.”

E. coli infections are not symptomatic for anywhere from three to 10 days. Chipotle believes the infected ingredients were served on Oct. 24. The restaurant said they would bring new ingredients in and test the meat, produce, and dairy for possible infections. According to Chipotle, no employees were sickened in the time period.

The limited menu available at Chipotle might have made the process harder for those trying to find the infection source, according to Dr. William Schnaffner, an expert on infectious disease at Vanderbilt University Medical Center.  Since most Chipotle menu items share ingredients, it is not easy to pinpoint a single ingredient as the source.

Food with low level contamination can produce a false negative. A vegetable with very little bacteria can test as fine, but still sicken an individual.